1. Field of Invention
This invention relates to non-electrical light sources and, in particular, to self-luminous signs and safety markers.
2. Brief Statement of the Prior Art
Tritium gas has been used for many years in the manufacture of self-luminous signs and safety markers, as well as in instrument panels and other military applications. Tritium gas is an unstable isotope of hydrogen and emits an electron upon decay of the atomic nucleus, thus producing low level beta radiation. The beta radiation has been used to excite certain phosphors which transform the radiated energy into photons of light. The current state of the art consists in sealing the tritium gas inside a glass capsule which has been coated on its interior surface with a suitable phosphor which emits photons of visible light. Since tritium has a "half life" of approximately 121/2 years, a tritium "lamp" may produce useful light in some cases for as much as 20 years.
Tritium lamps are available in several different configurations and sizes. They may be obtained in tubular form with a round cross-section or tubular form with a square or rectangular cross-section or in the form of a hollow disc or as a globe. These various shapes and sizes are used in a wide variety of signs and safety markers, all of which utilize at least one surface of their outer enclosure as the light-emitting surface.
Commercial quantities of tritium gas is available primarily as a by-product of nuclear powered generators. As a radioactive substance it is tightly controlled and regulated by government agencies, and is of course expensive. Any increase in the efficacy of its use is desirable from an economic standpoint and also as a means of conserving the available supply of a very limited commodity. Current state of the art usage of tritium lamps as safety signs and markers requires that the "lamp" be encased in a high-impact plastic housing to insure the integrity of the glass "lamp". While the glass surface of the lamp itself emits a uniform glow, the surface of the plastic housing usually does not, and when viewed from certain angles, appears to have uneven surface brightness. A uniform and increased surface brightness is desirable for both aesthetic and practical reasons. The aesthetic reason is obvious. The practical reason is to increase the usable angle of vision.
Recent improvements have been made in light collecting plastics by incorporating a fluorescent dye in transparent plastics such as polycarbonates and poly methyl methacrylates. These plastics absorb incident radiation in the ultraviolet and violet end of the visible light band of the electromagnetic spectrum, and fluoresce at a characteristic wavelength of the dye which has been incorporated within the plastic. The combined properties of internal reflection and fluorescence have been recognized to provide advantages for use of these plastics for advertising displays, decorative purposes, and solar panels.